In the case of low-orbit satellite communication systems, the use of a focusing system of the parabola type is not adequate. Specifically, in order to ensure the continuous tracking of nongeostationary satellites over their trajectory and to avoid the interruption of communication when said satellites are no longer in direct line of sight with the ground antenna, the latter must exhibit, at least during the period of switching from one satellite to another, two separate beams. Moreover, the angular coverage of the beams must be ensured over a very wide area.
To respond to these problems, it is possible to use a focusing system of the Luneberg lens type which, by virtue of its spherical symmetry, makes it possible to envisage a multitude of beams and the tracking of satellites over a wide angular sector by simple displacement of the transmission/reception sources in the focal surface of the lens. However, the practical embodiment of a Luneberg lens is complex and expensive. Consequently, in place of a Luneberg lens, it is possible to envisage the use of a homogeneous spherical lens.
A homogeneous lens exhibits a lower manufacturing cost. However, it does not allow perfect focusing of an incident plane wave. Specifically, aberration phenomena are noted at the level of the focal surface. In the case of a homogeneous lens, one no longer speaks of a focal point as in a focusing system constituted by a parabola or a Luneberg lens but of a focal spot, the focusing area being more extended.
Consequently, the exit focusing imperfections of a homogeneous lens render the design constraints of the associated primary source antenna more complex. The main function of the source antenna associated with the homogeneous lenses is therefore to take into account and to compensate as well as possible for the phase and amplitude distortions introduced by this imperfect focusing system.
Thus, the application of Robieux's theorem makes it possible to show that the efficiency of an antenna system comprising a primary source antenna and its associated focusing system is optimal when the electric field E and magnetic field H of the source antenna and of the focusing system are mutually conjugate. The distribution of the fields in the aperture of the source antenna must therefore be identical to that of the focusing system in amplitude and its phase response must be in phase opposition.
The present invention therefore relates to a source antenna which makes it possible to obtain a distribution of the fields in its radiating aperture and which superimposes as well as possible with that generated by the focusing system. When the focusing system is a system of parabola type, the solution conventionally used for the source antenna is a horn. However, in the case of source antennas such as horns, the technique generally employed to ensure the symmetrization of the E and H planes consists in the addition of transverse or longitudinal furrows or corrugations inside or outside the horn so as to modify the modal distribution of the electromagnetic fields at the level of the aperture of the horn. The corrugations in fact introduce higher hybrid modes into the guided structure at the level of the corrugations, which make it possible to harmonize the phase- and amplitude-response in the aperture of the horn.
However, when the focusing system is a homogeneous lens, the focusing being less effective than at the exit of a focusing system of conventional parabola type, this translates into a much more extended focusing area. Therefore, corrugated horns do not constitute the best solution in the case of a focusing system of the homogeneous lens type.
Consequently, the present invention proposes another solution for the source antenna constituted by a radiating aperture.